Understanding difference between hard dry and cured

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PostPosted: Thu Jan 10, 2019 5:37 am
More questions as I attempt to understand terminologies.

Some epoxy primer I have, saids hard dry in 2hrs, fully cured in 7 days, can be recoated without sanding up to 7 days, can be topcoated in minimum 1hr.

What actually occurs between the 2hrs and 7 days, that is what is it about the paint that allows it to stil be painted too in that 7 day period but after 7 days you cannot without sanding.

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Steve

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PostPosted: Thu Jan 10, 2019 9:47 am
Curing is a chemical process and not evaporation. The epoxy begins curing once mixed with the hardener and during the early stages goes from soft to sandable, however it does not reach full cure until much later.
During this cure time (different for each brand) you can apply filler or top coat without sanding. I believe most guys advise you to scuff/sand if the epoxy has been left more than 4 days just to be on the safe side. Also, if you spray another coat of epoxy during the open window it reactivates the epoxy.
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PostPosted: Thu Jan 10, 2019 10:31 am
:goodpost: Yep, all that and.....I had a finish chemistry class one time with a very "practical" professor that "dumbed down" stuff like this for us metal and wood shop guys. When materials use a hardener (as already explained) that is a catalyst that kicks off a molecular "handshaking." Each hand shake or cross link starts making the material a new harder/resistant material. During that cross linking you can still add more coating which will join in the process without any mechanical bonding (sanding) needed. Once the cross linking has stopped you can only bond by sanding.
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PostPosted: Thu Jan 10, 2019 12:38 pm
DarrelK wrote::goodpost: Yep, all that and.....I had a finish chemistry class one time with a very "practical" professor that "dumbed down" stuff like this for us metal and wood shop guys. When materials use a hardener (as already explained) that is a catalyst that kicks off a molecular "handshaking." Each hand shake or cross link starts making the material a new harder/resistant material. During that cross linking you can still add more coating which will join in the process without any mechanical bonding (sanding) needed. Once the cross linking has stopped you can only bond by sanding.


I'm sure if there were more educators like that professor, knowledge wouldn't be so difficult to grasp, hard to understand, or troubling to apply. Dumb down? Maybe? I like to think it becomes digestible? :goodjob: :goodpost:



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PostPosted: Thu Jan 10, 2019 1:15 pm
you MUST remember that temperature plays a BIG roll in the dry and cure time with epoxy. to cold and almost nothing happens, to warm and things happen rapidly, and in-between is just in-between. :)
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PostPosted: Thu Jan 10, 2019 1:35 pm
badsix wrote:you MUST remember that temperature plays a BIG roll in the dry and cure time with epoxy. to cold and almost nothing happens, to warm and things happen rapidly, and in-between is just in-between. :)
Jay D.


Yes, good point Jay.
The cut off temperature for cross linking (curing) to occur is somewhere below 60 degrees.
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PostPosted: Fri Jan 11, 2019 5:45 am
Thank you for the replies, good reading info.

What is the difference then between hard dry and cured?

The handshakes scenario makes way more sense to me :), whilst it’s still curing the handshakes are happening inside and around the skin, once the handshakes have stopped, then you have to key the surface to put the next layer on.

Is that becuase the subsequent coats soften the skin of the paint or the thinners within the fresh paint allow it to bite into the surface. I am assuming once it’s chmeically cured it cannot soften or penetrate into the original layer.

I have a few big pieces coming back from the blasters next week which will be epoxy primed first, my goal is to be in the cure window and follow up with raptor liner rather than have to sand the surface with 120grit.

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PostPosted: Fri Jan 11, 2019 9:39 am
"Is that because the subsequent coats soften the skin of the paint or the thinners within the fresh paint allow it to bite into the surface. I am assuming once it’s chemically cured it cannot soften or penetrate into the original layer."

Steve, yeah, you've got it.... And further....not sure how the other guys think on this here but I'd still scuff that epoxy before applying the raptor. I'm not sure that's needed but I wouldn't just assume the cross link chemistry between those two coating systems was necessarily achieving the same kind of bond a 2k or base coat would achieve. Maybe give raptor tech. line a call.....
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PostPosted: Fri Jan 11, 2019 10:07 am
From the Upol Tech Sheet:

Preparation
Q. Can RAPTOR be applied on top of a 2-part Epoxy Primer?

A. As long as the primer is fully cured, RAPTOR can be applied on top of a properly prepared and sanded 2-part epoxy primer.
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PostPosted: Fri Jan 11, 2019 10:22 am
Yep, that's what I was concerned about.....
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